1. Field of the Invention
The invention relates to the heating of continuously charged furnaces, and in particular to a method of heating furnaces intended to raise to a high temperature, as uniformly as possible, steel-making products which may have a large cross section, for example slabs, billets, blooms or ingots, and to a heating (or reheat) furnace of this kind.
2. Description of the Related Art
The temperature of steel-making products is raised in this way for example so that these products can be rolled, because steel is more malleable at high temperature and better lends itself to the operation.
The furnaces for which this method is intended may be beam-type furnaces, continuous pusher-type furnaces, and rotating-hearth furnaces in particular.
The invention also relates for example to furnaces for carrying out heat treatments "on the fly", particularly for part-finished or finished products (strip, tubes, wire, miscellaneous components).
Ideally, a furnace that performs well is a furnace which delivers a practically uniform temperature with good productivity, forming little scale (or oxides) on the surface, because scale, which is removed just before rolling, corresponds to a significant loss of material, and no adhering scale, thus avoiding the phenomena of "stress cracking" or burning of the products, and which produces a low amount of oxides of nitrogen and carbon dioxide.
The continuously charged furnaces to which the invention pertains generally stretch longitudinally between a product-charging end and a discharging end, the products being conveyed from one end to the other so that they pass right along the internal space of the furnace.
Along this internal space, these furnaces comprise, in succession, zones which fulfil different functions, sometimes immediately identifiable from the existence of internal partitions or particular roof profiles, but sometimes having no distinct physical demarcation.
More specifically, starting from the charging end, conventional furnaces of this type include, first of all, a portion which has no burners, then a portion which has air/fuel burners extending approximately as far as the discharge end.
The portion with burners thus comprises one or more heating zones, for example, from the upstream end in the downstream direction, a preheat zone, a heating zone proper, and an equalization zone near the discharge end from which the heated products are directed towards a rolling installation, for example; the flames developed by the burners allow the products in the furnace to be heated directly or indirectly using heat from the wall of the furnace. The essential method by which heat is transmitted is by radiation in the heating and equalization zones (accounting for more than 90%).